The present invention relates to a sputtering method and a sputtering apparatus for forming metallic films, insulating films or the like on substrates. For example, the present invention can be applied to a magnetic head such as GMR (Giant Magneto-Resistive element) including, for example, Cu films and Ni--Fe--Co films alternately located between a passivation film and a substrate.
Sputtering has been often carried out to form thin films of semiconductor, optical disks, electronic components, etc. in these days.
Hereinbelow, an example of a conventional sputtering method (for example, which is disclosed in Japanese Examined Utility Model Publication No. 2-43867) will be described with reference to FIGS. 6 and 7. In FIG. 6, reference numerals 1-3 represent a target, a cathode equipped with the target 1, and a substrate arranged opposite to the cathode 2 to which a film is to be formed by sputtering, respectively. Other reference numerals are: 4 a vacuum discharge pump for achieving a vacuum atmosphere in a vacuum chamber 5; 6 a gas feed system for supplying a sputtering gas to the vacuum chamber 5 while regulating a flow rate of the gas thereby to make a pressure in the vacuum chamber 5 to be a sputtering pressure; 7 a trigger gas feed system for triggering a discharge gas; 8 a pressure regulation valve for regulating the pressure in the vacuum chamber 5; 9 a power source for supplying electricity to the cathode 2 to generate plasma at a surface of the target 1, respectively.
The operation of a sputtering apparatus constituted as above will now be described. First, the interior of the vacuum chamber 5 is vacuumized into the order of 10.sup.-6 Torr by the vacuum discharge pump 4. Argon gas is then introduced into the vacuum chamber 5 by the gas feed system 6. The vacuum chamber 5 is adjusted to be a pressure of 2.times.10.sup.-3 Torr or so as a sputtering pressure, and then the pressure regulation valve 8 is fixed. The substrate 3 is set in the vacuum chamber 5 through a vacuum atmosphere reserve chamber or a transfer chamber, etc. Thereafter, as shown in FIG. 7, a direct current power source or a high frequency power source 9 is turned ON at a timing T11 thereby to apply electricity to the cathode 2. At the same timing, the trigger gas feed system 7 is opened to raise the pressure in the vacuum chamber 5 to be higher than the sputtering pressure, which brings about a discharge in the vacuum chamber 5. At a timing T12 when the discharge is detected by a discharge detection sensor such as a photo-detector or the like (when the discharge detection sensor is turned ON), the trigger gas feed system 7 is closed, so that the vacuum chamber 5 is returned to the sputtering pressure.. A film is thus formed on the substrate 3. In this case, the electricity applied to the cathode 2 reaches a set power at a timing T13. The direct current power source or high frequency power source 9 is turned OFF at a timing T15 when a film formation time terminates. The discharge is stopped. The discharge detection sensor is turned OFF at this time, that is, in a state not to detect the discharge. Then, the substrate 3 is transferred to the reserve chamber or transfer chamber or the like, and a fresh substrate 3 is set in the vacuum chamber 5. After the substrates 3 are transferred, the vacuum chamber 5 is returned to approximately 2.times.10.sup.-3 Torr as the sputtering pressure at a timing T16, and at the timing T16, the direct current power source or high frequency power source 9 is turned ON to supply electricity to the cathode 2. The trigger gas feed system 7 is opened at the same timing to make the pressure in the vacuum chamber 5 higher than the sputtering pressure. As a result, a discharge is brought about in the vacuum chamber 5. The trigger gas feed system 7 is closed at a timing T17 when the discharge detection sensor such as a photo-detector detects the discharge (is turned ON). The vacuum chamber is consequently returned to the sputtering pressure and a film is formed on the substrate 3. The electricity applied to the cathode 2 at this time reaches the set power at a timing T18. The discharge is stopped with the direct current power source or high frequency power source 9 being shut at a timing T20 when the film formation time expires. The discharge detection sensor is turned into a state not to detect discharging (is turned OFF). The substrate 3 is moved to the reserve chamber or transfer chamber, etc. in the vacuum atmosphere and a fresh substrate 3 is sent in the vacuum chamber 5. The above operation is repeatedly carried out afterwards. A pressure change when the substrate is exchanged to the reserve chamber or transfer chamber, etc. is not indicated in FIG. 7.
According to the above sputtering method, since the gas remains in the trigger gas feed system 7 while the trigger gas feed system 7 is closed, the gas is abruptly supplied at a moment when the trigger gas feed system 7 is opened at the timing T11. Therefore, the vacuum chamber 5 assumes 1.times.10.sup.-1 Torr or higher pressure, as shown in FIG. 7, which raises the necessity of a long time to reach a timing T14 when the vacuum chamber 5 returns to the sputtering pressure. An interface of the film to the substrate 3 becomes thick because of the higher pressure than the original sputtering pressure. Moreover, since a film formation speed generally decreases in a high pressure state, the film is formed virtually at a low speed as a whole. A change in time interval of discharges because of the exchange of substrates 3, etc. changes the amount of gas stagnation in the trigger gas feed system 7. That is, the pressure in the vacuum chamber 5 rises unstably when the trigger gas feed system 7 is opened, thereby rendering the timing T14 instable for the vacuum chamber 5 to return to the sputtering pressure. In addition, the pressure and electricity for starting the discharge vary due to a large pressure increase, in other words, the timing T12 starting the discharge becomes instable. As a result of this, a discharge duration (film formation time) varies, which leads to an instable film thickness. Besides, the film quality at the interface is instable. The conventional sputtering method has many issues as above.
In the case of sputtering to optical disks or the like, films have been formed in a short time, namely, in units of seconds. The aforementioned change in film quality and film thickness causes defective products and lowers a yield. For example, if the absolute value of the thickness of the film is unstable, the reflectance may make unstable to cause the signal strength to vary and thus the disk characteristics is lost and stored information can not be read out, and the step coverage may change not to recognize pits, and the heat conduction may change to change crystal condition. Additionally, if a film is formed in a case where the initial pressure is too high, the adhesive force of the film may be weaken and the refractive index may be adversely changed. Although a thermal filament method or the like is executed in many cases to trigger the discharge, the method requires a filament mechanism, a power source for a filament current and the like, making facilities complicate and expensive.